Field
This disclosure relates generally to crystal oscillators, and more specifically, to a crystal oscillator configured with a cross-coupled oscillation unit including diode loads having transistors, resistors, and a cross-coupled capacitors.
Background
A crystal oscillator (XO) circuit is a critical component of a radio frequency (RF) system and is used for reference frequency generation in transceivers. For these transceivers, responses to a frequency different from that to which the transceivers are tuned (spurious responses) are one of the challenging issues. The XO harmonics can easily couple to other RF circuits through multiple paths and appear at receiver and transmitter outputs. Compared to a single-ended structure, a differential XO circuit is desirable due to better immunities to interference and spurious responses.
Conventional differential XO circuit designs have used a negative conductance (−Gm) of a modified cross-coupled pair of transistors as an active device and have added high-pass filtering to avoid latching at low frequencies. For example, FIG. 1 is a schematic diagram of a conventional differential XO circuit 100. In FIG. 1, the differential oscillation is achieved using a cross-coupled pair of metal-oxide semiconductor field-effect transistors (MOSFETs) 120, 122 and a resonator 110. A supply voltage (VDD) couples to the drain terminals of the cross-coupled MOSFETs 120, 122 through resistor loads 140, 142. Two alternating current (AC) coupling capacitors 130, 132 are added to provide high-pass filtering and, therefore, close-to-zero DC gain. However, the lack of a low-impedance DC path in the XO circuit causes the circuit 100 to latch up rather than provide oscillation under certain conditions.